This invention relates to the production of deep cleaned coal by a physio-chemical cleaning and, more particularly, to a new and improved coal swelling technique to facilitate separation of inorganic impurities and sulfur compounds from coal.
There is a pressing need for an effective and economical method for cleaning coal which would encourage increased use of coal as an alternative utility energy source and meet air-quality standards without the use of flue gas desulfurization systems. Deep cleaned coal, containing less than 1% sulfur and 1% ash, not only can satisfy most current air-quality standards, but also is a potential alternative fuel in oil or gas-fired units. The low ash level, in particular, would also use of coal with minimal derating of equipment due to slagging, fouling, and erosion of heat transfer surfaces, thereby also improving the performance of coal combustion equipment.
Extensive research in deep coal cleaning is ongoing and uses either advanced physical or chemical cleaning approaches. Physical cleaning of coal employs mechanical grinding to liberate mineral impurities followed by selective separation to recover the cleaned product. Highly efficient comminution processes must be employed to obtain the extremely fine grinding needed for liberating mineral matter from the coal. In addition, high performance separation techniques are required for removing the fine ground mineral matter from the coal. The similarity of the surface and chemical characteristics of coal fines and mineral matter, especially pyrite, further complicates the separation, particularly as regards separation techniques that depend upon surface property differences for separation. Thus, the efficiency of physical cleaning depends on the degree of mineral liberation and the effectiveness of the selective separation technique. Usually, the more finely the coal is ground, the better the mineral liberation. Although ultrafine grinding (approximate maximum size of 10 microns) can help achieve maximum ash mineral liberation for most coals, it also can cause difficulties in downstream separation of coal fines without contamination by fine mineral particles and excessive Btu loss.
Existing advanced physical cleaning processes, with sophisticated separation techniques, such as selective oil agglomeration or selective flocculation procedures, can produce deep clean coal products containing less than 3% residual ash mineral content, but they all have to grind the coal down to the sub-micron particle size range before separation. The high energy consumption associated with ultrafine grinding, however, leads to an unacceptably high cost of production of the deep cleaned coal. It has been observed that the energy consumption for grinding coal to a size no greater than 10 microns is as high as 300 KWH/ton. Moreover, the inability of processes, such as selective oil agglomeration or selective flocculation procedures, to remove organic sulfur from coal limits the applicability of these advanced physical cleaning technologies to deep clean coal production.
Some chemical cleaning methods use chemical reagents to convert the solid mineral impurities into soluble or gaseous species which are then separated from the cleaned coal. Processing conditions which must be controlled include chemical concentration, temperature, pressure, and residence time. Difficulties in chemical cleaning of coal include maximizing the level of ash and sulfur reduction while minimizing volatile matter loss, undesirable side reactions, Btu loss, and operating costs.
While some existing advancing chemical cleaning processes can remove a high percentage of ash and a portion of organic sulfur, they also require intensive processing conditions. The TRW Gravimelt process, for example, can remove almost all the ash and up to 70% of the organic sulfur from coal with a molten caustic mixture of alkali metal hydroxide at 390.degree. C. for 2 to 4 hours. These conditions, however, may cause volatile matter loss. The Ames Lab Wet Oxidation Process requires pressure and temperature which result in non-selective oxidation reactions, causing heat loss and low efficiency in coal sulfur removal. Also available chlorinolysis processes involve multiple steps, including a high temperature dechlorination procedure (up to 700.degree. C.), which leaves a cleaned char product.